I don't know what this painting is of, but on doing a google search I found a lot of paintings from this time period of acts of beheading and I found it disturbing that many doing the beheading in these pictures were women.
After the abduction of the Sabine women by the neighboring Romans, the Sabines attempted to get them back - David depicts this episode here. The Sabine women are intervening to stop the bloodshed. Hersilia is throwing herself between her husband, the king of Rome, and her father, the king of the Sabines. David is using the subject to advocate the reconciliation of the French people after the Revolution. His increasingly simple style is inspired by Ancient Greece.